The present invention relates to a method for modulating the sedimentation rate of dry pellets which are used as fish feed and to piscicultural dry pellets with controlled desimentation rates.
As feeds for the culture of various seawater fishes such as yellowtail, sea bream, salmon, bastard halibut, striped jack, etc., freshwater fishes such as carp, trout, ayu or sweetfish, eel, landlocked salmon, tilapia, catfish, etc., and crustaceans such as kuruma prawn, black or jumbo shrimp, etc., there have been used live feeds such as sardine, horse and various mackerels, etc., moist pellets (Oregone moist pellets) prepared by granulating live feed-mash mixtures, and dry pellets prepared by a process of extruding a feed material based on fishmeal, gluten and starch.
Of these feeds, live feeds have the disadvantage that when they are minced and cast, the fishing grounds tend to be soiled. Moist pellets are very satisfactory feeds but since these are prepared by kneading a live feed with a mash and granulating the kneaded mixture, the production thereof requires a large-scale plant including a freezing plant, granulating machinery and so on, thus being procedure-wise complicated and time-consuming.
Therefore, dry pellets which are free from these disadvantages and amenable to automatic feeding have for some time been polularized. While some dry pellets do not contain oils or fats as ingredients positively added, recently oil-rich high-energy dry pellets containing 10 to 50 weight percent of oil and/or fat are attracting attention from the standpoint of improving the body weight gain of fish and the feed efficiency.
These dry pellets are generally manufactured by means of a single-screw or twin-screw extruding machine on the principle that the water, heat and pressure applied cause the starch component of the feed to swell as it is discharged from the extruder into the atmosphere.
The dry pellets thus manufactured are sometimes called expanded pellets (briefly, EP) in recognition of the above production mechanism and as the name implies, are low in specific gravity and, hence, float on the water after scattering. As may also be conjectured from the above mechanism involved, the higher the starch content or the higher the heat and pressure within the extruder, the greater is the tendency of pellets to swell.
With regard to the oil component, it is contributed, at least in part, by the main ingredient fishmeal but an oil or fat is sometimes added positively and the degree of expansion decreases with an increasing amount of the oil added. Thus, when the oil component is high, the degree of expansion is sacrificed and the pellets gain in specific gravity and tend to sink. To insure a sufficient degree of expansion, it has been attempted to increase the proportion of starch and/or intensify the heat and pressure conditions. However, this line of approach may beckon such objectionable outcomes as an excessively increased proportion of starch which is low in feed efficiency (because carnivorous fish do not digest starch well) and destruction of vitamins which are either inherent in the fishmeal or added positively.
On the other hand, the relationship between fish and the buoyancy or sedimentation property of feed pellets cannot be defined in universal terms. By way of illustration, young yellowtail, sea bream and other fishes demand relatively small-sized floating pellets in their youth but, as they grow, come to favor large-sized, sinking pellets and are even more fastidious in their choice according to seasons, favoring slowly sinking pellets in one season and more or less fast sinking pellets in another. Freshwater fishes, such as carp, trout and so on generally favor floating pellets, while crustaceans such as prawn and shrimp generally hunger for sinking pellets.
It might be possible, though to a limited extent, to meet such diversified demands by modifying the specific gravity and, hence, sedimentation rate of pellets through control of the quality and quantity of fishmeal and starch, the level of addition of oil/fat, the quantity of water added, and the extruder operating conditions such as granulation heat and pressure. However, many of these materials and operating conditions are narrow in the latitude of control and/or demand complicated procedures for control.
Thus, the conventional pellet manufacturing technology is not flexible enough in the control of the buoyancy or sedimentation rate of pellets.